1. Field of the Invention
The present invention relates to a method of recording data by radiating light beams, such as a laser beam onto an optomagnetic recording medium having a magnetic film.
2. Related Background Art
Recently, extensive studies have been made for optical memory elements as high-density, large-capacity memory elements using laser beams. An optomagnetic recording method received a great deal of attention as a rewritable recording method. The optomagnetic recording medium used in this method is expected to find use as a rewritable optical memory element.
As typical materials for constituting an optomagnetic recording layer of an optomagnetic recording medium used in such optomagnetic recording method, MnBi materials, garnet materials, rare earth-transition metal amorphous materials, and the like, are known. Since the MnBi materials have high Curie temperatures, they require a high-power laser for recording, and have many grain-boundary noise components. Therefore, high-S/N recording cannot be performed. Since the garnet materials have high light transmittances, they require a high-power laser for recording. The rare earth-transition metal amorphous materials have low Curie temperatures and relatively small light transmittances. Thus, they are expected to compensate for the formers' drawbacks.
A technique of this type will be described in more detail with reference to the drawings.
FIG. 1 is a sectional view illustrating a typical conventional optomagnetic recording medium.
In FIG. 1, a light-transmitting substrate 61 consists of a plastic material such as polymethylmethacrylate (PMMA), polycarbonate (PC), or the like, or a glass or the like, and normally has a planar shape such as a doughnut shape. An intermediate layer 62 consists of SiO, SiO.sub.2, AlN, ZnS, or the like. An optomagnetic recording layer 63 normally consists of a rare earth-transition metal amorphous material such as TbFe, GdTbFe, TbFeCo, or the like, for the above reasons. A reproduction/erase operation is performed as follows.
The recording medium is magnetized in a given direction perpendicular to the substrate 61, and a laser beam is radiated from the side of the substrate 61 to form a laser spot thereon. The direction of magnetization can be an arbitrary direction if it is constant. The laser beam radiated on the substrate 61 is transmitted through the substrate 61 and the intermediate layer 62, to reach the optomagnetic recording layer 63. As a result, light absorption occurs on an irradiated portion of the optomagnetic recording layer 63 irradiated with the laser beam and the temperature locally increases. Thus, the temperature of the irradiated portion reaches the Curie temperature, and its coercive force is decreased. At this time, if a bias magnetic field is applied to the portion of the optomagnetic recording layer 63 where the coercive force is decreased, in a direction opposite to the direction of magnetization, the direction of magnetization of that portion is inverted, and an inverted magnetic domain having a direction of magnetization different from that of a non-irradiated portion is formed thereon, thus recording data. Data can be erased as follows. A laser beam is again radiated on the recorded portion of the optomagnetic recording layer 63 to increase the temperature near the Curie temperature. A bias magnetic field is applied to the recorded portion in a direction opposite to that in the recording mode so as to restore the direction of magnetization to a state before recording. In the recording/erase operation, the film thickness of the intermediate layer 62 is selected to be a thickness that can effect a reflection preventive function with respect to a wavelength of the laser beam to be applied. Thus, an increase in temperature on the optomagnetic recording layer 63 can be effectively utilized for recording and erasing operations.
Data can be reproduced as follows. A laser beam with decreased power that cannot cause a temperature rise to the Curie temperature is radiated from the side of the substrate 61, and then the direction of magnetization of the recorded portion is read out by utilizing the magnetic Kerr effect.
As shown in FIG. 2, a multi-layered structure consisting of a high-coercive force layer 73 having a low Curie point and a high coercive force and a low-coercive force layer 72 a high Curie point and a low coercive force has been proposed in IEEE Transactions on Magnetics, Vol. MAG-17, No. 6, November 1981, and the like. In this structure, bits recorded on the high-coercive force layer 73 are transferred to the low-coercive force layer 72 by exchange interaction or magnetostatic interaction acting therebetween, in the same manner as in the prior art shown in FIG. 1, thereby recording identical data bits on these two layers. Reproduction utilizes the Kerr effect in the low-coercive force layer 72. In this manner, a layer corresponding to the recording layer (high-coercive force layer 73) and a layer corresponding to a readout layer (low-coercive force layer 72) are separately arranged, thereby improving recording sensitivity and read characteristics.
However, the conventional optomagnetic recording methods described above are binary digital recording methods. The directions of magnetization of bits correspond to "1" or "0" signals, respectively. In this method, a bit length or track pitch must be reduced in order to increase recording density, but such reduction is limited in favor of cross-talk prevention or due to optical limitations of beam spot size.
Studies have also been made for a method of performing analog recording. However, a C/N (carrier-to-noise) ratio is not satisfactorily high, and such a method cannot be used in practical applications.